Where the Scriptures are silent we should be silent also. Someone had the interpretation that Even knew the serpent and that s how the seed of Cain was formed but that is clearly conjecture and not scriptural fact. In the Old Mosaic law the offering of grain was a plausible offering to be given to God. It was the lack of faith in Cain s offering. Sometimes the best commentary of the Old Testament is the New Testament. For example in Hebrews 11 it tells us about the situation of Cain and Abel. Just because God questions in genesis doesn t mean that God doesn t know but rather he is trying to get Cain to confess. God knows exactly what I happening all the time but God is merciful to try and get us to confess our sins before Him. We always try to justify ourselves rather then just saying, I am wrong . There was much more a purer strain of humans in the beginning therefore marrying a sister would not cause such genetic defects as it would today. Methuselah almost lived for an entire millennium. Noah s father still lived at the same time that Adam was living. Methuselah died in the year of the flood. Enoch broke the chain, he was a man of faith because he walked with God. He has the testimony in Hebrews that Enoch pleased God. The father says of His son, this is my beloved Son who I am well pleased with. We please God in our trusting Him. There are other records of the flood in Egypt and Babylon etc. Some of them are gross accounts of this biblical truth. But this truth shows that man has a common creation and start.
